1. Field Of The Invention
The present invention relates to a recording medium for storing data and a reproduction apparatus for reproducing the data stored in the recording medium.
2. Description Of The Related Art
Recently, laser disks and video CDs (compact disks) have been input into practical use as optical recording media for storing multi-media data including moving picture data, audio data and sub-picture data.
Among these media, video CDs are used for storing moving picture data on a CD, having a memory capacity of about 600 megabytes and originally used for storing digital audio data, using MPEG. MPEG is a technology for compressing moving picture data at a high ratio. Many laser disks used in a variety of fields, for example, laser disks used in "karaoke" bars to show the singer the lyrics of the song are being replaced with video CDs.
Today, active studies are being conducted to improve the memory capacity of optical disks. In order to raise the memory capacity of optical disks, the diameter D of a light spot on the optical disk needs to be reduced. Where the wavelength of the laser light is .lambda. and the numerical aperture of the objective lens is NA, the diameter D of the light spot is in proportion to .lambda./NA. That is, the memory capacity can be higher as the wavelength .lambda. is smaller and the numerical aperture NA is greater.
When a lens having a greater numerical aperture is used, frame aberration caused by tilt, which is the relative angle of the optical axis of the optical beam with respect to the surface of the optical disk, increases as described in, for example, the U.S. Pat. No. 5,235,581. In order to prevent the frame aberration, the thickness of the transparent substrate needs to be reduced. A thinner substrate has a lower mechanical strength, which can be effectively compensated for by reinforcing the substrate with another substrate. The reinforcement is more effective when the two substrates have an identical thickness and are formed of an identical material.
Digital video disks (hereinafter, referred to as "DVDs") are also being developed today. For reading data from the DVD, a semiconductor laser for generating red light having a wavelength of as short as 650 nm and an optical system including an objective lens having a numerical aperture of as great as 0.6 mm are used. A DVD having a 120 mm diameter for storing 5 gigabyte data on one side has been proposed by using the above-mentioned semiconductor laser and optical system in combination with a substrate having a thickness of as small as 0.6 mm.
Memory disks having such a large memory capacity show their specific value in the field of multimedia, in which a plurality of moving picture data, a plurality of audio data, a plurality of graphic data and the like are reproduced through dialogue with the user.